1. Field of the Invention
The present invention relates to a light source device which generates a second-harmonic from laser light, using a second-harmonic generating element, and converts the second harmonic to a parallel beam of light filled with a bundle of rays up to a center portion of the beam.
2. Description or the Related Art
The nonlinear optical effect can be described as follows. when light is made incident upon a nonlinear optical material, there occurs a polarization proportional to a term of a higher degree than the square of the electric field of that light. The second harmonic is generated by this phenomenon.
Inorganic materials such as KH.sub.2 PO.sub.4 and LiNbO.sub.3 are examples of nonlinear optical materials. Organic materials typified by 2-methyl-4-nitroaniline (MNA) can also be used since they have large nonlinear optical coefficients.
In addition, wavelength converting elements for reducing in half the wavelength of a low-output laser beam, such as a semiconductor laser, by using the aforementioned nonlinear optical material as the second-harmonic generating element have been used. Such a wavelength converting element is designed to contain a fundamental wave, such as semiconductor laser light, for the harmonic at a high energy density and to extend the length of interaction with the harmonic.
For this reason, an optical waveguide type, for instance, is used as the form of the second-harmonic generating element. This optical waveguide type is so arranged that an elongated optical waveguide portion, for allowing light to be propagated by being contained within the same, is formed on a substrate and is covered with an overlayer thereon. However, in order to collimate the second-harmonic generated by the optical waveguide portion and the like, the optical waveguide must be structured in such a manner as to cope with the phase velocity of propagation of the second harmonic of the relevant wavelength. That is, the fundamental wave and the second harmonic wave must be phase-matched. To obtain this phase matching, various methods have been conceived. The simplest method known is a second-harmonic generating element using a Cerenkov radiation method.
In the Cerenkov radiation method, as shown in FIG. 6, a second harmonic generated from light being propagated through an optical waveguide portion 11 of an optical waveguide type second-harmonic generating element at a point A, enters base 12 and overlayer 13 at an angle .theta.. When the equiphase plane of a second harmonic generated at point B, in the direction of .theta., and the equiphase plane of the aforementioned second harmonic coincide with each other, a second harmonic wave emerges in the direction of the angle .theta.. The phase of the second harmonic wave matches with the phase of the fundamental wave when the refractive index of the substrate with respect to the fundamental wave is n.sub.s (.omega.), the refractive index of the waveguide portion is n.sub.G (.omega.), and the refractive index of the substrate with respect to the second harmonic is n.sub.s (2.omega.), insofar as the condition: EQU n.sub.s (2.omega.)&gt;n.sub.G (.omega.)&gt;n.sub.s (.omega.)
is met.
However, in the second-harmonic generating element of the optical waveguide type, since the second-harmonic is radiated from the optical waveguide portion having a small width to base, the light beam is crescent-like in cross section so that the light cannot be focused to a small spot. Hence, it is difficult to make use of this second harmonic in the writing and reading of an optical storage medium having fine pits such as an optical disk.
In contrast, since the second-harmonic generating element of an optical fiber type is axially symmetric, the second harmonic expands in an annular manner and can be converted to a parallel beam of light.
Accordingly, a light source device has been proposed which comprises a laser light source, an optical fiber-type second-harmonic generating element emitting a second harmonic from laser light emitting from the laser light source, and a collimator lens having a circularly symmetric, inclined surface and adapted to convert the second harmonic emergent from the second-harmonic generating element to a parallel beam of light, Japanese Patent Application Laid-Open No. 1-287531.
In accordance with the light source device having the above-described arrangement, if the laser light emitting from the laser light source is introduced to the optical fiber-type second-harmonic generating element to generate the second harmonic, the second harmonic expands from the end face of the optical fiber in the form of a wave having an axially symmetric and a conical equiphase plane.
FIG. 7 shows this manner, and the second harmonic expands as a conical beam B through a cladding 42 of an optical fiber 4. Accordingly, as the second harmonic is passed through a collimator lens having a circularly symmetric, inclined surface at least partially, it is possible to obtain a parallel beam of light of the second harmonic.
However, although the light emitting from the above-described light source device is capable of focusing the light to a spot, since the light is annular and the central portion thereof is missing, i.e. a doughnut-shaped light beam, paraxial approximation cannot be applied. Hence, there is a drawback in that this light source device is susceptible to the effect of spherical aberrations of members constituting an optical system, such as lenses. In addition, when the light source device is used for an optical disk device, it is necessary to specially redesign the structure of an optical detector for collimating an astimatism signal so as to detect a tracking error, Japanese Patent Application Laid-Open No. 2-15434.
Accordingly, it is preferable, if it is possible, to generate a light beam which is filled with a bundle of rays up to a central portion of the beam.
Hence, it is conceivable to obtain a beam filled with a bundle of rays up to a central portion of the beam by grinding the distal end of a conical ends 10 and by bringing it into contact with the emergent end face of the fiber, as shown in FIG. 8. However, this method has a problem in that damage is liable to occur of the ground surface of the fiber at the time of alignment between the fiber 4 and the lens 10.